Chemical Constitution of Saline Solutions. 291 
At the temperature of 100° a basic salt separated; and while this decomposition 
occurred, the black lines thickened, and those bands which form a group in the 
green became confused. Only four measurements could be made at this tempera- 
ture on account of the turbidity of the liquid. 
Iodide and bromide of didymium were prepared, but, as they readily decom- 
posed, they were unsatisfactory salts to work with. 
Didymium Sulphate.—A finely crystallized specimen, sparingly soluble in water. 
A saturated solution was made at 20°. It was photographed with the four-prism 
spectrograph, the plate being so adjusted, that the zero of the scale used in measur- 
ing the spectra was at the edge of the plate, and the red end of the spectra carefully 
focussed. 
The illumination was sunlight, and the solution was contained in a test-tube 
half an inch in diameter. This salt is not very soluble, as it required more than 
forty times its weight of water to dissolve it at 20°. 
ABSORPTION BAND, I. 
r r 
ANG XN, 5 . 5822 to 5750 
ANG 100% 6 . . 5834 to 5750 
ABSORPTION BAND, II. (not so strong as I.). 
r r 
ING PO, - 5 . 5833 to 5308 
IMG WOO", : . 9333 to 5808 
Rays transmitted and the spectrum terminates— 
Xr 
NS WO, . 9 oo SR 
NeTOU, 5 o oo SB 
The spectrum begins at } 6198. The second band is much weaker than the 
first, and is less distinct at 20° than when the solution is hot. 
It will be seen from these measurements and by inspection of more recent 
photographs, that at 100° the absorption bands have widened slightly, this 
being particularly noticeable in the first one, and slightly in the second, where it 
is seen to be shifted, apparently, but this is due to the band having widened on 
the weaker or less retrangible side when the solution was hot. The most remark- 
able change in the spectrum caused by rise of temperature is the increased 
absorption of the ultra-violet rays, which in the cold solution were entirely 
transmitted as far as the sun’s spectrum could be photographed. 
Having, in June last, the opportunity of photographing the absorption spectra 
of neodym-ammonium nitrate and praseodym-ammonium nitrate from small 
